Post SMTP would not touch another plugins settings. It is more than likely a part of Mandrill’s deactivation routine in that it clears up after itself.
Far too often removing a plugin leaves behind orphan data that after a while can build up. Hence why it’s always best to take a backup of the database and plugins before you remove or disable something.
As for Post SMTP picking up Mandrill’s settings, whilst I can’t speak for Yehuda (the post SMTP developer), that would be difficult to maintain. Given you should ideally remove the Mandrill plugin (or any other mailer/SMTP type plugin) before installing Post SMTP (to avoid conflicts), if Mandrill was written correctly, it would have removed its settings so there would be nothing for Post SMTP to read.
However, one suggestion you could make to the Post SMTP developer is the ability to import settings. You could then export your settings from Mandrill (and perhaps later other plugins) and then run an import tool within Post SMTP. That does however assume that Mandrill and perhaps other plugins have a settings export tool! But in Post SMTP’s defence, we’re not talking a ton of variables to import, so for the sake of 2-4 variables, is it worth the hassle?
But just to summarise, I don’t believe Post SMTP removed the settings, I’m 99.9% certain Mandrill probably cleaned up after itself.